The present invention relates to the art of coated metal strip such as strip used for architectural materials, gasoline tanks and filter housings; however, the invention has broad applications and relates to various coating alloy compositions based upon tin and zinc and several novel method concepts used therein such as continuous hot dipping and applying an intermediate metal layer prior to hot dip coating. This invention is particularly applicable to the coating of thin stainless steel strip by applying an ultra thin layer of a metal, such as nickel, onto the strip as the strip is continuously progressing toward a molten bath of tin and zinc coating metal which is substantially free of lead, wherein the strip is hot dip coated to form an intermetallic layer and the invention be described with reference to the use; however, the invention has broader applications and can be used by applying an ultra thin intermediate layer of other metal, particularly tin, chromium, nickel and copper prior to hot dip coating.
Over the years, architectural materials, such as metal roofing systems and metal siding systems, made of pliable metals in various sheet gauge thicknesses have been used. Metals such as carbon steel, stainless steel, copper and aluminum are the most popular types of metal used for such architectural materials. The term "stainless steel" is used in the technical sense and includes the use of chromium, plated or alloyed, with a ferrous base.
Carbon steel architectural metal materials were commonly treated with corrosion-resistant coatings to prevent rapid oxidation of the metal surface, thereby extending the life of the materials. A popular corrosion-resistant coating for carbon steel is a terne coating. Terne coating of stainless steel and copper is also produce, but is much less prevalent than carbon steel due to the natural corrosion-resistant properties of stainless steel and copper. Terne coating has been the predominate and the most popular coating for carbon steel materials due to its relatively low cost, ease of application, corrosion-resistant properties and desirable colorization during weathering. Terne coated carbon steel is used for gasoline tanks, roofing and building materials and for various other products.
When the terne coated steel sheets are assembled into a roof covering, adjacent sheet edges are folded over one another to form a seam, typically a standing seam, and the seam is usually soldered vis-a-vis the terne coating to produce a waterproof joint. Today, the terne coated steel sheets can either be preformed or be formed at the job site onto roofing pans with bent edges which abut edges of adjacent pans which are then pressed or rolled into the seam. Similarly, caps, cleats, etc. are likewise formed from the terne coated sheet. In addition to providing for soldering of the seams, the terne coating inhibits rusting or oxidation of the metal sheet which would otherwise occur over time.
Terne or terne alloy is a term commonly used to describe an alloy containing about 80% lead and the remainder tin. The terne alloy is conventionally applied to the metals by a hot dip process wherein the metal strip is immersed into a molten bath of terne metal by a continuous or batch process. The terne coating inhibits the formation of ferrous oxide on the metal thus preventing corrosion and extending the life of the coated metal. The corrosion resistant properties of the terne alloy are primarily due to the stability of elemental lead and tin and the lead-tin oxide which forms from atmospheric exposure.
Although terne coated metals have excellent corrosion-resistant properties and have been used in various applications, terne coated materials have recently been questioned due to environmental concerns. Terne coated metals contain a very high percentage of lead. Although the lead in terne alloys is stabilized, there is concern about leaching of the lead from the terne alloy. Environmental and public safety laws have been recently proposed and/or passed prohibiting or penalizing the user of materials containing lead. Because the terne alloy contains a very high percentage of lead, materials coated with terne have been prohibited in various types of usages or applications such as aquifer roofing systems. The concern of lead possibly leaching from the terne coating has made such coated materials inadequate and/or undesirable for several types of building and manufacturing applications. When terne is used in the automotive field, such as gasoline tanks, the components are eventually scrapped and the terne coated parts are discarded into land fills. The discarding of such parts has raised recent environmental concerns especially with respect to underground water sources. Consequently, terne coated automotive parts are in the process of being replaced. The prevailing wisdom in the business has concluded that there is no viable coated steel substitute for terne coated parts. Consequently, gasoline tanks and various other parts are being designed as plastic with all of the associated economical and environmental problems associated with plastic materials.
Another disadvantage of terne coated materials is the softness of the terne layer. As noted, terne coated metal sheets are commonly formed into varying shapes. The machines that bend the metal sheets periodically damage the terne coating during bending process. The terne coating is susceptible to damage due to the abrasive nature of the forming machines.
The terne alloy has a further disadvantage in that the newly applied terne is very shiny and highly reflective. As a result, the highly reflective coating cannot be used on buildings or roofing systems such as at airports and military establishments. The terne coating eventually loses its highly reflective properties as the components within the terne coating are reduced (weathered); however, the desired amount of reduction takes approximately 1 1/2 to 2 years when the terne coating is exposed to the atmosphere, thus requiring the terne metals to be stored over long periods of time prior to being used in these special areas. The storage time is significantly prolonged when the terne coated materials are stored in rolls and the rolls are protected from the atmosphere.
A tin coating electroplated on a carbon steel strip has been recently suggested for use for gasoline tanks and related automotive components; however, such a coating is expensive to manufacture and is not necessarily successful for a coated strip drawn into such uses.
Tin coating of carbon steel is a well-known process for use in the food industry. However, in the specialized art of architectural materials, a tin coating for architectural materials has not been used until just recently as disclosed in U.S. Pat. No. 5,314,758.
The most popular process for applying a tin coating to carbon steel for use in the food industry is an electrolysis process. In an electrolysis process, the coating thickness is very thin and typically ranges between 3.8.times.10.sup.-4 to 20.7.times.10.sup.-4 mm (1.5.times.10.sup.-5 to 8.15.times.10.sup.-5 in.). Furthermore, the equipment and materials needed to properly electroplate the metal materials are very expensive and relatively complex to use. The expense of applying an electroplated tin coating and the limited obtainable thicknesses of the tin coating are a disadvantage for using such a process for building and roofing materials and in the automotive field. Such processes create an extremely thin layer with a network of small pinholes making the strip generally unacceptable. Such electroplated strip may have a base flash layer and/or a cover coating to overcome the pinhole problems inherent with an electroplating process.
After over a decade of attempting to develop a substitute for terne coated steel for gasoline tanks and related disposable components, the present solution has been to electroplate metals such as tin or tin and zinc. Such processes create an extremely thin layer with a network of small pinholes making the strip generally unacceptable. Before the research and development project of Applicants, the steel industry had no commercially acceptable process for producing a thin ferrous strip having a corrosion-resistant coating constituting an alloy which was essentially free of lead.
At this time, in 1995, the tin industry is still proposing and experimenting with electroplating of tin with protective layers because the process is believed to be the only way to continuously coat steel. Hot-dipping of tin has been generally ignored even though a hot-dip process for applying the tin coating may be used. It has been found that when the metal strip is not properly prepared, the coating is not properly applied to the materials and minute areas of discontinuity in the tin coating occur resulting in non-uniform corrosion protection. This is especially a problem when the tin is applied to stainless steel materials by a hot-dip process. Tin is not electroprotective to steel under oxidizing conditions. Consequently, discontinuities in the tin coating result in the corrosion of the exposed metal. Although the exposed stainless steel corrodes at a significantly slower rate than standard carbon steel, the stainless steel will eventually corrode especially in high corrosive environments, i.e. petroleum, receptacles, marine products.
Prior to Assignee's application Ser. No. 000,101, the concept of coating stainless steel with a corrosive-resistant material had proven of limited success and coating stainless steel with tin by a hot-dip process had repeatedly been unsuccessful using conventional hot-dip processes as discussed above. Prior to Assignee's application Ser. No. 000,101, the only process which semi-successfully coated stainless steel with tin was the complex and expensive electroplating process. The thickness of the tin plate was limited to a very thin thickness of not more than 20.7.times.10.sup.-4 mm (8.15.times.10.sup.-5 in.). The limited tin coating thickness resulting from electroplating limited the uses and life of the tin plated materials.
Tin coatings have the further disadvantage of having a highly-reflective surface. As a result, materials coated with a tin coating cannot be used in an environment where highly-reflective materials are undesirable until the coated materials are further treated (i.e. paint) or the tin is allowed time to oxidize.
Coating architectural materials and other metal strip with zinc metal, commonly known as galvanization, is another popular metal treatment to inhibit corrosion. Zinc is a highly desirable metal to coat architectural materials because of its relatively low cost, ease of application (i.e. hot-dip application) and corrosion resistance. Zinc is also electroprotective to steel under oxidizing conditions and prevents the exposed metal, due to discontinuities in the zinc coating, from rapidly corroding. This electrolytic protection extends away from the zinc coating over exposed metal surfaces for a sufficient distance to protect the exposed metal at cut edges, scratches, and other coating discontinuities.
With all of the advantages of using zinc, zinc coatings have several disadvantages that make it undesirable for many types of building applications and for automotive components. Although zinc coatings will bond to many types of metals, the formed bond is not strong and results in the zinc coating flaking off the building materials. Flaking of zinc or zinc oxide in a gasoline tank will clog the gasoline lines and filters. Further, when using fuel injection systems, the small particles of zinc or zinc oxide will disable the injectors. Such problems are unacceptable in the automotive field. Thus, galvanized strip is common, but, not used for gasoline tanks. Zinc does not bond well on standard stainless steel materials. Zinc does not form a uniform and/or thick coating in a hot-dip process for stainless steel. As a result, discontinuities of the coating are usually found on the stainless steel surface. Zinc is also a very rigid and brittle metal and tends to crack and/or flake off when the materials are formed on site, i.e. press fitting of roofing materials or when gasoline tank components are drawn. When zinc begins to oxidize, the zinc coating forms a white powdery texture (zinc oxide). The popular grey, earth tone color is not obtained from pure zinc coatings.
Electroplating a tin and zinc mixture onto a steel sheet is disclosed in Japanese Patent Application No. 56-144738 filed Sep. 16, 1981. The Japanese patent application discloses the plating of a steel sheet with a tin and zinc mixture to form a coating of less than 20 microns thick. The Japanese patent application discloses that after plating pin holes exist in the coating and subject the coating to corrosion. The pin holes are a result of the crystalline layer of a tin and zinc mixture which slowly forms during the plating process. The charged tin and zinc atoms in combination with the atomic structure of the atoms and formed crystal structure of a tin and zinc mixture prevents a uniform coating from being formed on the plated steel sheet. Consequently, the crystalline depositions must be covered with a chromate or phosphoric acid to fill the pin holes and prevent immediate corrosion.
The electroplating of the tin and zinc mixture onto the plated articles does not form an intermetallic layer between the article and the plated tin-mixture. Only when high temperature levels are obtained to melt the tin and zinc does an intermetallic layer form. As discussed in Assignee's application Ser. No. 165,085, the molten tin in a tin coating alloy interweaves with the surface atoms of the coated article. This intermetallic layer forms a strong bond between the metal strip and the molten alloy. The intermetallic layer also has excellent corrosion-resistant properties. Because the Japanese Patent Application No. 56-144738 does not produce an intermetallic layer during electroplating, the Japanese patent application discloses that a preplated layer of nickel, tin or cobalt on the steel sheet surface is needed so that the plated tin and zinc mixture will adhere to the steel sheet. Such electroplating techniques, as disclosed in the Japanese patent application, cost a tremendous amount of time and money and do not produce a commercially successful product. The Japanese patent application creates a network of pinhole, as does any electroplating process; therefore, the strip, when drawn, creates large areas exposing the base metal. Thus, in the manufacture of gasoline tanks, steel would be exposed directly to the stored liquid fuel and rapidly corrode.
The coating of steel articles by a batch hot-dip process with a tin, zinc and aluminum mixture is disclosed in U.S. Pat. No. 3,962,501 issued Jun. 8, 1976. The '501 patent discloses that the tin, zinc and aluminum mixture resists oxidation and maintains a metallic luster. The '501 patent discloses that the coating is applied by immersing a steel article into the molten alloy bath and subsequently withdrawing the steel article. The '501 patent also discloses that a molten tin-zinc alloy bath containing 3-97% zinc is very susceptible to oxidation at the surface thus producing viscous oxides which causes severe problems with the batch hot dip process of immersing the steel articles into the molten alloy, hold the article in the molten alloy bath for a period of time and subsequently removing the steel article from the molten alloy. Further, while the steel article is in the molten alloy, a large amount of dross is produced which results in non-uniformity of the coating and the formation of pin holes. The '501 patent discloses that the addition of up to 25% aluminum to the tin and zinc mixture inhibits dross formation during immersion of the steel article, prevents Zn--Fe alloy formation and reduces the viscous oxide formation on the molten bath surface. The '501 patent does not suggest the coating of continuous metal strip, nor does it teach the use of a continuous hot dip coating process which resolves the viscous oxide problem and dross formation problem. The continuous hot-dip process of a strip material subjects all the surfaces of the strip to a uniform residence time in the molten alloy to produce a relatively uniform coating and coating thickness on the continuously moving strip. A batch process as disclosed in the '501 patent subjects the surface of the article to differing residence times in the molten alloy during immersion and removal of the article into the molten alloy. The only way to overcome the disadvantage of such a process is to hold the article in the molten metal for a prolong time as suggested by the '501 patent. This differing of residence time in the molten alloy produces differing coating thicknesses and coating properties on the coated article or dictates a long holding time in the metal. The '501 patent also discloses the formation of a highly reflective coating which cannot be used in many building applications.
The treatment of a steel sheet by plating tin and zinc is disclosed in U.S. Pat. No. 4,999,258. The '258 patent discloses a steel sheet plated with a layer of tin and subsequently applying a layer of zinc such that the ratio of the zinc to tin is 2-30%. The tin and zinc plated layers are then heated until the zinc alloys with the tin. The tin and zinc plated coatings are plated to form a very thin layer. The tin is applied at 0.2-1.0 g/m.sup.2 and the zinc is applied at 0.01-0.3 g/m.sup.2. Due to the very thin coating thickness, the flow heating of the tin and zinc plating layers only requires 2-5 seconds for tin-zinc alloying. The '258 patent also discloses that when less than 1% zinc is used, the beneficial effect of the zinc is null; however, when more than 30% zinc is used, the coating will rapidly corrode under adverse environments. The '258 patent also discloses that a nickel plated layer is preferably applied to the steel sheet prior to applying the tin and zinc plated layers to improve corrosion resistance. The heat treated tin-zinc layer can be further treated by passivating with a chromate treatment to further improve corrosion resistance. The '258 patent does not teach the use of a hot dip coating process, nor does it teach the use of a continuous hot dip coating process which resolves various process problems such as viscous oxide and dross formation. The '258 patent is limited to the producing of a very thin tin-zinc coating. The tin-zinc coating is not a homogeneous coating since the zinc diffuses into the tin during heating to form a strata of layers. Equilibrium of the tin and zinc concentrations is not obtained during the short heating process. Such a thin coating is also highly susceptible to tearing when the coated metal material is formed into products such as automotive products and roofing products.
A continuous process for electroplating a carbon steel strip is disclosed in U.S. Pat. No. 5,203,985. The '985 patent discloses that nickel is electroplated on a continuously moving strip of carbon steel. After the carbon steel has been nickel plated, the plated strip is hot dip coated with molten zinc. The '985 patent is absent any teachings concerning a two phase (binary) tin and zinc alloy and the advantages of using such a two phase (binary) coating. Prior to the present invention, a tin and zinc coating was applied to a metal surface primarily by an electroplating process. As discussed above, such plated tin and zinc coatings were commercially unacceptable.
Due to the various environmental concerns and problems associated with corrosion-resistant coatings applied to metal architectural materials and automotive products, there has been a demand for a coating which is easily and successfully applied to materials that protect the materials from corrosion, does not have a highly-reflective surface subsequent to application and is applied by an economical process.